Semiconductor and other delicate substrates may be electrostatically gripped in air or vacuum to minimise slippage while being accelerated during robotic transfer or alignment. Gripping combined with gas backfill on the rearside of substrates is also used to increase machine processing rates through the improved temperature control afforded by thermal conductivity of the backfill gas. Often such vacuum operation employs radio-frequency (rf) excitation to generate plasma discharge bombardment of the substrate, requiring the gripper to be at high rf and induced dc voltages. Such gripping has been performed using high voltage generators mounted remotely from the gripper, then connected by wires, rf filters, feedthroughs, slip rings, and other direct electric connection methods to the gripper. An example of such connections is given in U.S. Pat. No. 5,646,814 “Multi-Electrode Electrostatic Chuck” by S. Shamouilian, S. Broydo and M. Birang. Such connections limit the possible motions of the gripper, the ability of the remote electronics to sense signals at the gripper potential, and also the available rf excitation levels and frequencies due to the restricted range of rf filters. In addition bulky filter and rotary coupling hardware is often required at the crowded gripper site.
Vacuum feedthroughs, rotary slip rings, and bending or rotating wiring are often used to couple power from the high voltage drive to a gripper inside a vacuum chamber. Vacuum feedthroughs can exhibit arcing from high voltage points in vacuum feedthroughs as the pressure of gas passes through intermediate easily ionized regimes. Slip rings have been noted for their unreliability of slip ring contact, as noted in U.S. Pat. No. 5,994,788 “Device for the contactless transfer of signals between two vehicle parts” by K. Dobler, E. Zabler, A. Dukart and T. Hermann. An example of slip ring reliability improvement is given in U.S. Pat. No. 3,938,029 “Low noise dc power supply system for electronics on a rotating assembly” by P. B. Wagner and J. W. Telfordm further type of low noise slip ring using orientation-sensitive liquid mercury contacts is made by Mercotac Inc. of Carlsbad, Calif. 92009 USA. However these slip ring improvements are incompatible with high speed signal transmission and vacuum operation, respectively.
A further inconvenience of rf filters and long wiring connections relates to capacitive sensing of substrate grip status, used in the control of the electric field levels at the surface of electrostatic grippers. Examples of such sensing are given in U.S. Pat. No. 5,103,367 “Electrostatic chuck using ac field excitation” by C. M. Horwitz and S. Boronkay; and U.S. Pat. No. 5,325,261 “Electrostatic chuck with improved release” by C. M. Horwitz. Sensitivity to substrate grip status varies when rf filter frequency settings are altered or when cables move, requiring readjustment of sensing parameters or resulting in inconsistent substrate sense signal levels.
If it were possible to place the drive electronics module at the gripping device, and to power and control it remotely, such limitations of wiring and filter connections would be removed. Remote powering of a gripping power supply may be accomplished via light beams, radio waves, heat radiation, mechanical coupling, and other such means of energy transfer. Only some of these powering methods are sufficiently efficient to be used in vacuum applications; for example photocells are rarely more than 50% efficient even for monochromatic illumination. Such less efficient methods would be viable in non-vacuum applications where air cooling is available.
Operation of gripping devices and their high voltage drive unit in vacuum, which does not allow gas conductive or convective removal of heat can result in undesirable overheating of the electronics if energy transfer is not efficient or the remote electronics module dissipates a high heat load relative to its heat radiating size. In addition space is limited in most vacuum chambers so a small size of all components is required, which further constrains the allowable heat load. Electrostatic gripping thus requires an efficient method of remotely powering a high voltage electrostatic grip module. In addition control of the module via a two-way communication link is desirable to both initiate actions and obtain feedback on system status.
A further and more popular method of efficient energy transfer uses rotary coupling transformers. Some recent examples of the use of such transformers to couple energy and information between two isolated regions are described in U.S. Pat. No. 5,691,687 “Contactless magnetic slip ring” by H. Kumagai; the Dobler patent referenced above; U.S. Pat. No. 6,032,546 “System for transferring electrical power between non-contacting elements in relative motion” by N. Stone; U.S. Pat. No. 6,012,736 “Vehicle steering column control system” by J. E. Hansen, R. E. Hubbell, W. J. Janutka, B. T. Pier, S. A. Reid, W. L. Rutchik; U.S. Pat. No. 5,824,891 “Method and apparatus for efficiently phase modulating a subcarrier signal for an inductively coupled transformer” by M. B. Monson; U.S. Pat. No. 5,856,710 “Inductively coupled energy and communication apparatus” by J. S. Baughman and K. C. Ross. These methods describe systems with various degrees of power transfer efficiency and communication effectiveness between the coupled circuits. The power coupling devices used to isolate against both radio frequency and dc high voltages should preferably employ a simple construction with a relatively large gap of 2-3 mm which can be filled with high voltage insulation and an electrostatic shield. Thus constructions such as in the Kumagai and Stone patents referenced above which rely on small gaps such as 0.0005 to 0.005 inches in the case of Stone, or such as the Dobler patent referenced above using dished and interleaved magnetic elements, would not be applicable.
It is thus one of the aims of this patent to provide means of removing the need for direct wire connections and rf filters while permitting efficient power transfer for substrate gripping with a power drain sufficiently low that overheating in vacuum does not occur.
Another aim of this patent is to provide means of communication between substrate and control unit so that full status monitoring, accurate substrate position sensing, and control are maintained.
Another aim is to provide power coupling means with sufficiently large gap between the coupled elements to allow rf isolation and electrostatic shielding to be performed. Such a power coupling could also be used to couple across a vacuum window material, eliminating the need for a wired vacuum feedthrough.
Another aim is to provide a non-contact power coupling means that is sufficiently compact to fit inside rotary bearings, or that allows linear motion, between moving robotic segments.